I've applied dozens and dozens of skins - all brands (BSE, Zagg, Phantom Skinz, Clearfreak, etc etc). If you know what you're doing, and do it properly, you can safely power on the device within an hour or two.
However, I try to apply the skin in the evenings, where I have the luxury of shutting the phone off for 4-6 hours, and then insert the battery and turn it back on. Always works well and never a problem.
Key is if it's your first or second install, allow for more time to dry out, as you may use a lot more water than necessary. 12 hours is good enough, 24 hours is if you're a newbie to allow for extra safety margin.